A radio wave propagation characteristic estimation system (radio wave propagation simulator) is employed to assist the arrangement for a base station, a parent machine, or the like in a wireless communication system, and the optimization of parameters of the arranged base station and parent machine, or the like. The radio wave propagation simulator assesses a received power, a delay spread, or the like at an optional reception point to decide an installation side of a due transmitting station, so that the overall efficiency can be enhanced by reducing the number of the base stations to be arranged. Further, deciding an optimal parameter of the transmitting station enables a network quality of the wireless communication system to be enhanced.
The radio wave propagation simulation is largely divided into a simulation employing a statistical technique and a simulation employing a deterministic technique. The former statistical technique is a technique of giving an equation for estimating the propagation power loss (hereinafter, simply referred to as a propagation loss) with the arguments of a distance, a frequency, and so on to statistically determine the parameters on the basis of a large amount of data resulted from the actual measurements of the propagation loss. The details of the statistical technique are disclosed, for example, in Non-patent document 1 (Yosio Hosoya (supervisor), Radio Wave Propagation Handbook, REALIZE INC., 1999). On the other hand, the latter deterministic technique is a technique of pre-imitating propagation circumstances being estimated (a structure, an object, etc. that exert an influence upon the propagation of the radio wave) over a computer, deterministically calculating an influence that is exerted by these structure and object upon the radio wave radiated from an antenna, and estimating the radio wave state at the reception point. The deterministic technique is inferior to the statistical technique from a viewpoint of the operational processing amount, whereas the deterministic technique is higher in an estimation precision owing to deterministically taking an influence of the propagation circumstances into consideration. In particular, the statistical technique is difficult to apply, and rather, the deterministic technique is desirably applied in the circumstances where a lot of the structures and objects that exert an influence upon the radio wave propagation exist, for example, in a city, in a room, or the like.
There exists a raytracing method as a technique that is often employed, out of some deterministic techniques. The raytracing method is a method of, by considering that the radio wave radiated from an antenna is a collection of a number of radio wave rays, synthesizing the rays reaching to an observatory point, and obtaining the propagation loss and the amount of a delay on the premise that each ray is reflected and is transmitted repeatedly on the geometrical optics, and is propagated. Furthermore, the raytracing method is largely divided into a raylaunching method, and an imaging method. The details of the raylaunching method and the imaging method are disclosed, for example, in Non-patent document 1 and JP-P1997-33584A.
The raylaunching method is a technique of, successively tracing a truck of the ray on the assumption that the ray discretely radiated at each constant angle from a transmitting antenna is propagated while it is repeatedly reflected at and transmitted into the structure or the object. In a case of precisely estimating the propagation by employing the raylaunching method, an effect of diffraction other than the reflection or the transmission is desirably considered. As a technique of estimating the effect of diffraction by employing the raylaunching method, there exists the technique disclosed in JP-P2004-294133A. This technique is a technique of, at the time that the rays pass near an edge of the structure, allowing the rays equivalent to the diffracted wave to be generated at the above edge. The loss caused by the diffraction varies depending upon the direction at which the diffracted wave propagates after an occurrence of the diffraction, thereby necessitating generating a plurality of the rays as a diffracted wave component after an occurrence of the diffraction. This causes the problem that the number of the rays, which should be processed with an occurrence of one-time diffraction, increases, and thus the operational processing amount increases to that extent.
On the other hand, the imaging method is a technique of deciding a reflection and transmission path of the ray connecting between the transmitting/reception points by obtaining an imaging point against the reflection surface. The imaging method, which is capable of searching a strict propagation route of the ray between the transmitting/reception points, enables a higher estimation precision to be realized as compared with the raylaunching method. However, an increase in the number of the structures that are considered, and an increase in the maximum number of times of the reflection/transmission/diffraction cause the problem that the operational processing amount increases drastically. In particular, in a case of evaluating the radio wave propagation characteristic from a transmission point in a two-dimensional plane, the imaging method necessitates searching the propagation route of the ray at each reception point, whereby the operational processing amount furthermore increases to that extent. As a technique of reducing the operational processing amount of the imaging method, there exist, for example, the techniques disclosed in JP-P1997-33584A, JP-P1997-119955A, JP-P1997-153867A, JP-P2003-318842A, and JP-P2005-72667A.
The technology of each of JP-P1997-33584A, JP-P1997-119955A, JP-P2003-318842A, and JP-P2005-72667A, out of these patent documents, is for reducing the operational processing amount that is accompanied by a search for the propagation route of the ray, by reducing the number of the structures that are considered at the time of estimating the propagation, or simplifying a shape of the structure. For example, the technique of JP-P1997-33584A is for selecting and filing in advance a building existing in the road and the intersection in a street microcell constituting a service area along the road by arranging the base stations on the road, and estimating the propagation only by considering the above building. The technique of JP-P1997-119955A is for rectilinearly decomposing the road into the node subelements based upon road data indicative of the position of the road in the street microcell similar to the foregoing. The technique of JP-P2003-318842A is for pre-specifying the minimum value and the maximum value of the propagation delay time, and estimating the propagation by considering only the structure which causes the propagation delay time that is between the minimum value and the maximum value. The technique of JP-P2005-72667A is for searching a transmission point side prospect structure and a reception point side prospect structure, and estimating the propagation by considering only the above structure. These techniques could sacrifice the estimation precision in exchange of a reduction in the operational processing amount because the number of structures that are considered at the time of estimating the propagation is reduced, or a shape of the structure is simplified. Further, there is the case that a sufficient reduction in the operational processing amount cannot be attained depending upon the propagation circumstances.
On the other hand, the technique of JP-P1997-153867A is for pre-identifying presence or absence of the building existing between the transmission point and the reception point, obtaining the propagation loss by considering only a direct wave when the building is absent, and applying the imaging method only when the building is present, thereby to calculate the components of the reflected wave/transmitted wave/diffracted wave. The direct wave dominates in a case of where no obstructed view is afforded between the transmission point and the reception point, whereby at this reception point, the components of the reflected wave/transmitted wave/diffracted wave are not considered, thereby allowing a reduction in the operational processing amount to be realized. However, in a case of estimating the radio wave propagation characteristic in the city that is overcrowded with the buildings, almost all of the observatory points are not in prospect. Thus, applying the imaging method for all these points inevitably increases the operational processing amount.    Non-patent document 1: Yosio Hosoya (supervisor), Radio Wave Propagation Handbook, REALIZE INC. 1999.    Patent document 1: JP-P1997-33584A    Patent document 2: JP-P2004-294133A    Patent document 3: JP-P1997-119955A    Patent document 4: JP-P1997-153867A    Patent document 5: JP-P2003-318842A    Patent document 6: JP-P2005-72667A